Organs and Tissues for Human Movement
The question of which mechanisms are controlled and how the nervous system control the movement this has been investigated extensively and discussed at practically every stage of the history of motor control studies [e.g., Bernstein 1967; Granit, 1970; Arbib, 1980; Stein, 1982; Latash, 1993]. One important reason for controversies is the difference in understanding the word control. Human can control virtually any variable characterizing voluntary movement: joint angle (position), joint torque (force), movement speed, and accuracy. A systematic search for the “independently controlled” variables is connected to the classic work of Norbert Wiener ; however, the pioneering, yet very well defined conclusions come from Bernstein . He was the first to address the motor control system as a “black box” with a virtually unknown internal structure that must control an effector apparatus of multiple links and degrees of freedom [Latash, 1993]. His experimental studies were based on mapping of input and output variables of the motor system. The conclusions are the following:
the control system is a hierarchical structure with several levels;
feedback loops connect the lower levels with the higher ones in order to tune the descending (efferent) commands;
time delays in the feedback loops require combining feedback and predictive, open-loop modes of control;
the number of degrees of freedom in a motor system is always excessive, and the process of control can be regarded as overcoming the ambiguity caused by redundant degrees of freedom.
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